Portable electronic devices such as smart phones, tablet computers, laptop computers, and digital cameras usually consume a large amount of electricity. Therefore, nowadays, these electronic devices typically use a high-capacity dedicated lithium-ion (Li-ion) battery pack or lithium-ion polymer (Li-Po) battery pack, rather than primary cells (or disposable batteries) that are not rechargeable, or rather than nickel-metal hydride (Ni-MH) batteries that have a small rated capacity. Nowadays, in the field of some electronic devices such as portable flashlights that consume a large amount of electricity and that have a short product lifecycle, have been launched new products that are in compliance with 18650 standards of most-widely used cylindrical lithium-ion batteries.
Although electronic flash for cameras suffer very high power consumption, they still use a conventional battery in which commercial R6 (or AA) cells are connected in series, rather than a high-capacity dedicated battery pack. This is because the product lifecycle of camera flashes is relatively long (for example, 10 years or more) and compatibility with commercial batteries should be considered. For the same reason, camera flash manufacturers have restricted compatible batteries for their camera flashes to only four types: 1.5 V alkaline primary batteries; 1.5 V lithium primary (lithium iron disulfide or LiFeS2) batteries; 1.2 V nickel-metal hydride (Ni-MH) secondary batteries; and 1.2 V low self-discharge (LSD) Ni-MH secondary batteries.
An alkaline battery, which is one type of primary cell, is vulnerable to low temperature conditions. For example, the discharge capacity of an alkaline battery is reduced to about 60% of the rated capacity (about 2500 mAh) at −10° C. and reduced to about 20% of the rated capacity at −20° C. Furthermore, all the rated capacity (2500 mAh) can be fully used under a low drain condition of about 25 mA, but only less than half of the rated capacity (for example, 1000 mAh) can be used under a high drain condition of about 1000 mA. In other words, alkaline batteries are unsuitable for camera flashes because camera flashes are usually used under outdoor low temperature conditions and have very high power consumption. On the other hand, lithium primary batteries can be used stably at a rated capacity of about 3000 mAh even under low temperature and high drain conditions. However, the consumer price of lithium primary batteries is about eight times that compared to alkaline batteries, and thus are very expensive. In addition, because the above-mentioned two kinds of primary batteries cannot be recharged or reused, they are neither cost effective nor environment-friendly.
High-capacity nickel hydride batteries, one type of secondary batteries, have a rated capacity of about 2700 mAh, which is substantially equal to that of alkaline batteries or lithium primary batteries. However, high-capacity nickel hydride batteries have a high self-discharge rate, and frequently experience voltage depressions known as memory effect under high drain conditions. Therefore, at present, high-power consumption flashes rarely use high-capacity nickel hydride batteries. On the other hand, there are low self-discharge nickel hydride batteries that are free of the above-mentioned problems. However, these low self-discharge nickel hydride batteries have a problem of a low rated capacity of about 2000 mAh, which is approximately 26% lower than that of conventional high-capacity nickel hydride batteries. In order to supplement the lack electricity of low self-discharge nickel hydride batteries, camera flash manufacturers additionally provide customers with a quick recycling battery pack that can increase a supply voltage by which one R6 standards battery can be connected in series with battery compartment, or they separately sell an external battery pack in which six or eight R6 standards batteries are connected in series. This, however, results in an increase in cost and weight of camera equipment, which is undesirable.
Alkaline batteries or lithium primary batteries that are generally used for camera flashes have a rated voltage of 1.5 V, an open circuit voltage of 1.65 V, and an end of discharge voltage of 1.1 V. When four alkaline batteries or four lithium primary batteries are connected in series, they supply a voltage ranging from 4.4 to 6.6 V. On the other hand, when five alkaline batteries or five lithium primary batteries are connected in series by additionally using a quick recycling battery pack, they supply a voltage ranging from 5.5 to 8.25 V. On the other hand, lithium-ion batteries have a rated voltage of 3.6 to 3.7 V, a maximum charging voltage of 4.2 V, and a discharge cut-off voltage of 2.8 V. Therefore, it is impossible to apply cylindrical lithium-ion batteries with 14500 standards (the same as those of R6) to a 4-series (4S) circuit of a four-cell battery compartment in a camera flash. However, when lithium-ion batteries or lithium-ion polymer batteries each with 14500 standards are connected to be a 2-series & 2-parallel (2S2P) circuit, a voltage ranging from 5.6 to 8.4 V can be supplied. On the other hand, lithium iron phosphate batteries (LiFePO4) have a rated voltage of 3.2 V, a maximum charging voltage of 3.6 V and a discharge cut-off voltage of 2.8 V. When lithium iron phosphate batteries are connected to be a 2-series & 2-parallel (2S2P) circuit, a voltage ranging from 5.6 to 7.2 V can be supplied. Therefore, in such a case, lithium-ion batteries, lithium-ion polymer batteries, or lithium iron phosphate batteries can be used for a camera flash. In addition, lithium-ion batteries have 2 to 3 times more specific energy and energy density than nickel hydride batteries. Furthermore, lithium-ion batteries have high charge/discharge efficiency (90% or higher) compared to nickel hydride batteries having charge/discharge efficiency of about 65%. When a lithium-ion battery and a nickel hydride battery are the same in volumes or specifications, the weight of a lithium-ion battery is only half the weight of a nickel hydride battery.
However, all lithium secondary battery packs designed and marketed thus far are prismatic types or pouch types having an elongated flat shape. Or packaged two or more cylindrical type lithium-ion bare cells (each contained in a metal can) are simply connected to a protection circuit module (PCM) and fixed in an outer case. Therefore, it is impossible to insert and install those lithium secondary battery packs into a four-R6-standards-cell battery compartment of a camera flash. Furthermore, a cylindrical pouch-type bare cell having a Jelly roll-type electrode assembly has not yet been developed, manufactured, or marketed, which is needed to produce a lithium secondary battery pack that can fit snugly into a two-cell or four-cell (R6 standards cells) battery compartment of the electronic devices.